1. Field of the Invention
This invention relates to an engine control system for an automotive vehicle engine which controls purging fuel vapors supplied from a fuel tank into an intake system of the engine while controlling the engine to rotate at a target speed of rotation during running idle.
2. Description of Related Art
Typically, engines, especially vehicle engines, are controlled to run idle at a specified target speed of rotation by feedback control and learning control. Utilization is made of a value learned based on a control value in the idle speed feedback control as a correction value to control the idle speed of rotation of the engine in the idle speed learning control. Specifically, in the idle speed learning control, the engine is controlled to run idle at a target speed of rotation by means of correcting and controlling, for example, the amount of intake air bypassing a throttle valve with the feedback control value and the learned value.
On the other hand, it is typical for the engine to purge fuel vapors into an air intake system of the engine. The fuel vapors are usually stored in a canister and drawn into a surge tank forming part of the air intake system where they are mixed with fuel for burning whenever predetermined purge conditions are satisfied. A purge valve is provided between the canister and the air intake system and controlled to open, permitting the fuel vapors to be drawn into the air intake system together with fresh air introduced into the canister.
While on-idle fuel vapor purge may be performed, various constraints are imposed on controlling the speed of rotation of the engine, which are always undesirable. For example, purging fuel vapors causes a change in engine speed of rotation. For this reason, determination of the learned value is suspended during the on-idle fuel vapor purge as taught by, for example, Japanese Unexamined Patent Publication No. 5-202815.
When the canister is filled and saturated with fuel vapors, there occurs an escape of fuel vapors from the canister into the atmosphere. In recent years, in order to prevent or reduce significantly an escape of fuel vapors into the atmosphere, a new technology has been proposed to purge fuel vapors into the air intake system from the canister as frequently as possible so that the canister has storage capacity sufficient to store fuel vapors drawn from the fuel tank. From this view point, there is a demand for a sufficiently long period of time for an on-idle fuel vapor purge. If, however, the on-idle fuel vapor purge is performed too frequently, missing the opportunity of determining the learned value is encountered in the idle speed learning control.